Imagine the excitement of the first scientist who observed cells! This discovery was made in 1665 by the English physicist Robert Hooke, who used a primitive microscope (Fig. 3.1) to examine thin slices of cork found in stoppered wine bottles:
I took a good clear piece of Cork, and with a Penknife sharpen'd as keen as Razor, I cut a piece of it off, and thereby left the surface of it exceedingly smooth, then examining it very diligently with a Microscope, methought I could perceive it to appear a little porous . . . these pores, or cells . . . were
indeed the first microscopical pores I ever saw, and perhaps that were ever seen, for I had not met with any Writer or Person, that had made mention of them before this ... I had with the discovery of them, presently hinted to me the true and intelligible reason of all the Phaenomena of Cork.
Hooke compared the boxlike compartments he saw to the surface of a honeycomb and is credited with applying the term cell to those compartments. He also estimated that a cubic inch of cork would contain approximately 1,259 million such cells. What Hooke saw in the cork were really only the walls of dead cells, but he also saw "juices" in living cells of elderberry plants and thought he had found something similar to the veins and arteries of animals.
Two physicians, Marcello Malpighi in Italy and Hooke's compatriot Nehemiah Grew in England, along with Anton van Leeuwenhoek, reported for 50 years on the organization
of cells in a variety of plant tissues. In the 1670s, they also reported on the form and structure of single-celled organisms, which they referred to as "animalcules."
After this period, little more was reported on cells until the early 1800s. This lack of progress was mainly due to imperfections in the primitive microscopes and the crude way in which tissues were prepared for microscopic examination. Microscopes and tissue preparations both slowly improved, however, and by 1809, the famous French biologist Jean Baptiste de Lamarck had seen a wide enough variety of cells and tissues to conclude that "no body can have life if its constituent parts are not cellular tissue or are not formed by cellular tissue." In 1824, René J. H. Dutrochet, also of France, reinforced Lamarck's conclusions that all animal and plant tissues are composed of cells of various kinds. Neither of them, however, realized that each cell could, in many cases, reproduce itself and exist independently.
In 1831, the English botanist Robert Brown discovered that all cells contain a relatively large body that he called the nucleus. Soon after the discovery of the nucleus, the German botanist Matthias Schleiden observed a smaller body within the nucleus that he called the nucleolus. Schleiden and German zoologist Theodor Schwann were not the first to understand the significance of cells, but they explained them more clearly and with greater insight than others before them had done. They are generally credited with developing the cell theory, beginning with their publications of 1838 to 1839. This theory, in essence, states that all living organisms are composed of cells and that cells form a unifying structural basis of organization.
In 1858, another German scientist, Rudolf Virchow, argued persuasively in a classic textbook that every cell comes from a preexisting cell ("omnis cellula e cellula") and that there is no spontaneous generation of cells. Virchow's publication stirred up much controversy because there had previously been a widespread belief among scientists and nonscientists alike that animals could originate spontaneously from dust. Many who had microscopes were thoroughly convinced they could see "animalcules" appearing in decomposing substances.
The controversy became so heated that in 1860, the Paris Academy of Sciences offered a prize to anyone who could experimentally prove or disprove spontaneous generation. Just two years later, the brilliant scientist Louis Pasteur of France was awarded the prize. Pasteur, using swan-necked flasks, demonstrated convincingly that boiled media remained sterile indefinitely if microorganisms from the air were excluded from the media.
In 1871, Pasteur proved that natural alcoholic fermentation always involves the activity of yeast cells. In 1897, the German scientist Eduard Buchner accidentally discovered that the yeast cells did not need to be alive for fermentation to occur. He found that extracts from the yeast cells would convert sugar to alcohol. This discovery was a big surprise to the biologists of the time and quickly led to the identification and description of enzymes (discussed in Chapter 2), the organic catalysts (substances that aid chemical reactions without themselves being changed) found in all living cells. This also led to the belief that cells were little more than miniature packets of enzymes. During the first half of the 20th century, however, further advances were made in the refinement of microscopes and in tissue preparation techniques. Many structures and bodies, besides the nucleus, were observed in cells, and the relationship between structure and function came to be realized and understood on a much broader scale than previously had been possible.
Our investigation of life is greatly assisted by various types of microscopes that can magnify our images of cells and tissues up to hundreds or even thousands of times their actual size. A better understanding of cell structure and function is also provided by preparing in different ways the tissues that are to be examined. While light microscopes, similar to the one used by Hooke in 1665, provide basic information about cell structure and some of the bodies within cells, the development of electron microscopes has revealed detailed images of tiny structures within cells.
Light microscopes increase magnification as light passes through a series of transparent lenses, presently made of various types of glass or calcium fluoride crystals. The curvatures of the lens materials and their composition are designed to minimize distortion of image shapes and colors.
Light microscopes are of two basic types: compound microscopes, which require most material being examined to be sliced thinly enough for light to pass through, and dissecting microscopes (stereomicroscopes), which allow three-dimensional viewing of opaque objects. Microscopic organisms such as bacteria and protozoans often consist of a single cell and are thin enough to be viewed without being sliced. The best compound microscopes in use today can produce magnifications of up to 1,500 times under ideal conditions. Many dissecting microscopes used in teaching laboratories magnify up to 30 times, but higher magnifications are possible with both types of microscopes. Light microscope magnifications of more than 1,500 times, however, are considered "empty" because resolution (the capacity of lenses to aid in separating closely adjacent tiny objects) does not improve with magnification beyond a certain point. In general, when using a compound light microscope, one can distinguish organelles (bodies within cells) only if they are 2 micrometers or larger in diameter. In this chapter, the structures discussed that are most commonly observed with a light microscope include cell walls, nuclei, nucleoli, cytoplasm, chloroplasts, and vacuoles. Light microscopes (Fig. 3.2) will continue in the foreseeable future to be useful, particularly for observing living cells.
Since the 1950s, the development of high-resolution electron microscopes has resulted in observation of much greater detail than is possible with light microscopes.
Figure 3.2A A compound light microscope.
Instead of light, electron microscopes use a beam of electrons produced when high-voltage electricity is passed through a wire. This electron beam is directed through a vacuum in a large tube or column. When the beam passes through a specimen, an image is formed on a plate toward the base of the column. Magnification is controlled by powerful electromagnetic lenses located on the column.
Like light microscopes, electron microscopes are of two basic types. Transmission electron microscopes (Fig. 3.3A) can produce magnifications of 200,000 or more times, but the material to be viewed must be sliced extremely thin and introduced into the column's vacuum so that living objects can't be observed.
Scanning electron microscopes (Fig. 3.3B) usually do not attain such high magnifications (3,000 to 10,000 times is the usual range), but surface detail of thick objects can be observed when a scanner makes the object visible on a cathode tube like a television screen. The techniques for observation with electron microscopes have become so refined that even preserved material can appear exceptionally lifelike, and high-resolution, three-dimensional images can be obtained.
In 1986, the Nobel Prize in physics was awarded to two IBM scientists, Gerd Binnig and Heinrich Rohrer, for their invention in 1982 of a scanning tunneling microscope. This microscope uses a minute probe that "tunnels" electrons upon a sample. As the probe is moved across the surface, its height is continually adjusted to keep the flow of electrons constant, and the fluctuations in height are recorded to produce a map of the sample surface. Without doing any damage to the probed area, this microscope reproduces an image of such high magnification that even atoms can become discernible. The probe can scan areas barely twice the width of an atom and theoretically could be used to print on the head of an ordinary pin the words contained in more than 50,000 single-spaced pages of books. Early in 1989, the first picture of a segment of DNA showing its helical structure was taken with a scanning tunneling microscope by an undergraduate student associated with the Lawrence Laboratories in northern California. Several variations of this microscope, each using a slightly different type of probe, have now been produced. Significant new discoveries by cell biologists using one or more of all three types of microscopes in their research have become frequent events.
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Figure 3.3A A transmission electron microscope. (Courtesy LEO Electronenmikroskopie GmbH, Germany)
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.